<!-- Any copyright is dedicated to the Public Domain.
     http://creativecommons.org/publicdomain/zero/1.0/ -->
<!doctype html>

<html>
  <head>
    <meta charset="utf-8"/>
    <title>WebGL editor test page</title>

    <script id="shader-vs" type="x-shader/x-vertex">
      precision lowp float;
      attribute vec3 aVertexPosition;
      uniform float uDepth;

      void main(void) {
        gl_Position = vec4(aVertexPosition, uDepth);
      }
    </script>

    <script id="shader-fs-0" type="x-shader/x-fragment">
      precision lowp float;

      void main(void) {
        gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
      }
    </script>

    <script id="shader-fs-1" type="x-shader/x-fragment">
      precision lowp float;

      void main(void) {
        gl_FragColor = vec4(0.0, 1.0, 1.0, 1.0);
      }
    </script>
  </head>

  <body>
    <canvas id="canvas" width="128" height="128"></canvas>

    <script type="text/javascript;version=1.8">
      "use strict";

      let canvas, gl;
      let program = [];
      let squareVerticesPositionBuffer;
      let vertexPositionAttribute = [];
      let depthUniform = [];

      window.onload = function() {
        canvas = document.querySelector("canvas");
        gl = canvas.getContext("webgl", { preserveDrawingBuffer: true });
        gl.clearColor(0.0, 0.0, 0.0, 1.0);

        initProgram(0);
        initProgram(1);
        initBuffers();
        drawScene();
      }

      function initProgram(i) {
        let vertexShader = getShader("shader-vs");
        let fragmentShader = getShader("shader-fs-" + i);

        program[i] = gl.createProgram();
        gl.attachShader(program[i], vertexShader);
        gl.attachShader(program[i], fragmentShader);
        gl.linkProgram(program[i]);

        vertexPositionAttribute[i] = gl.getAttribLocation(program[i], "aVertexPosition");
        gl.enableVertexAttribArray(vertexPositionAttribute[i]);

        depthUniform[i] = gl.getUniformLocation(program[i], "uDepth");
      }

      function getShader(id) {
        let script = document.getElementById(id);
        let source = script.textContent;
        let shader;

        if (script.type == "x-shader/x-fragment") {
          shader = gl.createShader(gl.FRAGMENT_SHADER);
        } else if (script.type == "x-shader/x-vertex") {
          shader = gl.createShader(gl.VERTEX_SHADER);
        }

        gl.shaderSource(shader, source);
        gl.compileShader(shader);

        return shader;
      }

      function initBuffers() {
        squareVerticesPositionBuffer = gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER, squareVerticesPositionBuffer);
        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
           1.0,  1.0, 0.0,
          -1.0,  1.0, 0.0,
           1.0, -1.0, 0.0,
          -1.0, -1.0, 0.0
        ]), gl.STATIC_DRAW);
      }

      function drawScene() {
        gl.clear(gl.COLOR_BUFFER_BIT);

        for (let i = 0; i < 2; i++) {
          gl.bindBuffer(gl.ARRAY_BUFFER, squareVerticesPositionBuffer);
          gl.vertexAttribPointer(vertexPositionAttribute[i], 3, gl.FLOAT, false, 0, 0);

          gl.useProgram(program[i]);
          gl.uniform1f(depthUniform[i], i + 1);
          gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);
        }

        window.requestAnimationFrame(drawScene);
      }
    </script>
  </body>

</html>
